This invention relates to a vacuum pump, particularly to a turbomolecular vacuum pump which has a rotor supported in the pump housing on magnetic bearings and which further has two emergency (auxiliary) bearings.
The purpose of the emergency bearings is to prevent potentially damaging changes in the position of the rotor from a desired position. The reasons for radial rotor excursions are usually jars or lateral blows against the pump. Axial deviations from the desired position are caused, for example, by air rushing into the rotor zone or into the attached vessel to be evacuated. In vacuum pumps where the pump rotor is supported by electromagnetic bearings, further reasons for positional deviations from the desired position of the rotor may be a defect in the regulation of the coil current or other breakdown which interferes with the magnetic flux.
Emergency bearings for molecular or turbomolecular pumps are typically dry-running bearings since grease or oil adversely affect the quality of the vacuum generated by the pump. By using lubricated bearings, the advantage of the magnetic bearings, that is, an absolute freedom from lubricants and thus an absence of hydrocarbons would be surrendered. Since the service life of dry-running bearings is 0 limited, it is a requirement that the rotor, after a shift from the desired position has occurred in which the emergency bearing takes over, reassumes its desired position as rapidly as possible to thus avoid an excessive stress on the emergency bearings. In tests with prototypes, however, it has been repeatedly observed that the rotor, after lateral jars against the pump, tended to "stick" to the emergency bearings.